Catalytic gas phase oxidation processes of propylene with molecular oxygen has been widely adopted for industrial production of acrylic acid. In particular, two-stage oxidation processes comprising the first oxidation step of catalytically oxidizing propylene at gas phase to form acrolein and the second oxidation step of catalytically oxidizing the resulting acrolein at gas phase to produce acrylic acid are in the mainstream.
Acrylic acid is broadly utilized as a starting material for its esters, polymers and the like, and is an important industrial product. In these years demand for a water absorbent resin prepared from acrylic acid as the starting material is increasing, and in consequence demand for acrylic acid also is increasing.
A water absorbent resin is a water-swellable and water-insoluble polyacrylic acid having crosslinked structure, which can be obtained by crosslinking polymerization of acrylic acid and/or salt thereof as the base monomer (preferably at least 70 mol %, in particular, at least 90 mol %) using further about 0.001-5 mol % (to the acrylic acid) of a crosslinking agent and about 0.001-2 mol % to the acrylic acid of a radical polymerization initiator, followed by drying and pulverization of the product. The resin absorbs at least three times, preferably 10-1,000 times, its own weight of pure water or physiological saline solution to form water-insoluble hydrogel containing no more than 25 mass %, preferably no more than 10 mass %, of water-soluble component (water-soluble substance).
On the other hand, propylene, the starting material, also is a starting material of polypropylene, acrylonitrile and the like, and the need therefor is yearly increasing. In consequence, it is predicted that the propylene supply as the starting material of acrylic acid will become unable to catch up with the demand and there is a possibility of propylene shortage. With the view to cope with this, investigations for production process of acrylic acid using cheap and readily available propane as the starting material are vigorously made in recent years.
As to production processes for acrylic acid from propane as the starting material, various proposals have been made, such as oxidation of propane to directly produce acrylic acid, or first producing propylene by simple dehydrogenation or oxidative dehydrogenation of propane and producing acrylic acid from the resulting propylene by the two-stage oxidation process. None of those proposals is yet satisfactory for industrial scale working, however, and improvements are desired.
For example, as for the process comprising oxidative dehydrogenation of propane to produce propylene and the two-stage oxidation of the propylene to produce acrylic acid, various trials are made including suppressing propane conversion to a relatively low level in order to maintain high propylene selectivity in the oxidative dehydrogenation step and after the subsequent production of acrylic acid from the so formed propylene via acrolein in the presence of unreacted propane, recycling the unreacted propane for re-use.
JP 2000-502719A discloses that use of air as the oxygen source involves a possibility for the nitrogen in the air to adversely affect the recycling of the discharged gas, and hence it is preferable to use an oxygen source whose oxygen content is at least 90%.
Whereas, JP 2002-523387A proposes to use air as the source of the oxygen supply and to separate at least a part of the molecular nitrogen which is contained in the recycled gas. Also JP 2002-523389A and 2002-523390A propose to use modified air having a less nitrogen content and more oxygen content than those of air, as the source of oxygen supply, and to separate at least a part of the molecular nitrogen contained in the recycled gas.